Untangling Twinning

Maureen L. Condic

Description

Scientists and philosophers have long struggled to answer the questions of when human life begins and when human life has inherent value. The phenomenon of identical (monozygotic) twinning presents a significant challenge to the view that human life and human personhood begin at conception. The fact that a single embryo can split to generate two (or more) genetically identical embryos seems to defy the notion that prior to splitting an embryo can be a single human individual. In Untangling Twinning, Maureen Condic looks at the questions raised by human twinning based on a unique synthesis of molecular developmental biology and Aristotelian philosophy. She begins with a brief historical analysis of the current scientific perspective on the embryo and proceeds to address the major philosophic and scientific concerns regarding human twinning and embryo fusion: Is the embryo one human or two (or even more)? Does the original embryo die, and if not, which of the twins is the original? Who are the parents of the twins? What do twins, chimeras, cloning, and asexual reproduction in humans mean? And what does the science of human embryology say about human ensoulment, human individuality, and human value? Condic's original approach makes a unique contribution to the discussion of human value and human individuality, and offers a clear, evidence-based resolution to questions raised by human twinning. The book is written for students and scholars of bioethics, scientists, theologians, and attorneys who are involved in questions surrounding the human embryo.

Sujets

• Médecine
• Gynécologie et Obstétrique
• Medical
• Embryology
• Ethics

Informations

• Publié par: University of Notre Dame Press
• Date de parution: 28 février 2020
• Nombre de lectures: 0
• EAN13: 9780268107079
• Langue: English

Extrait

UNTANGLING TWINNING
NOTRE DAME STUDIES IN MEDICAL ETHICS AND BIOETHICS
O. Carter Snead, series editor

The purpose of the Notre Dame Studies in Medical Ethics and Bioethics series, sponsored by the de Nicola Center for Ethics and Culture, is to publish works that explore the ethical, cultural, and public questions arising from advances in biomedical technology, the practice of medicine, and the biosciences.
UNTANGLING TWINNING

What Science Tells Us about the Nature of Human Embryos
MAUREEN L. CONDIC
University of Notre Dame Press
Notre Dame, Indiana
University of Notre Dame Press
Notre Dame, Indiana 46556
undpress.nd.edu
Copyright © 2020 by the University of Notre Dame
All Rights Reserved
Published in the United States of America
Library of Congress Cataloging-in-Publication Data
Names: Condic, Maureen, author.
Title: Untangling twinning : what science tells us about the nature of human embryos / Maureen L. Condic.
Other titles: Notre Dame studies in medical ethics and bioethics.
Description: Notre Dame, Indiana : University of Notre Dame Press, [2020] | Series: Notre Dame studies in medical ethics and bioethics | Includes bibliographical references and index.
Identifiers: LCCN 2019054557 (print) | LCCN 2019054558 (ebook) | ISBN 9780268107055 (hardback) | ISBN 9780268107086 (adobe pdf) | ISBN 9780268107079 (epub)
Subjects: MESH: Twins | Beginning of Human Life—ethics | Embryonic Development | Bioethical Issues | Chimerism—embryology
Classification: LCC RG133.5 (print) | LCC RG133.5 (ebook) | NLM WQ 235 | DDC 176—dc23
LC record available at https://lccn.loc.gov/2019054557
LC ebook record available at https://lccn.loc.gov/2019054558
This e-Book was converted from the original source file by a third-party vendor. Readers who notice any formatting, textual, or readability issues are encouraged to contact the publisher at [e-mail]
This book is dedicated to Joseph Yost, who has supported me with critical scientific discussion, tireless devotion, and a never-failing Irish sense of humor.
CONTENTS

List of Illustrations
List of Tables
Acknowledgments
Note on Presentation of Citations in the Text
Introduction: Human Embryos and Human Individuals
1 When Does Human Life Begin?
2 Totipotency
3 What Is an Embryo?
4 Twinning and the Beginning of Human Life
5 Philosophical Concerns regarding Twinning
6 Philosophical Concerns Raised by Human Chimerism
7 Why Scientists Are Confused about Embryos
8 The Embryo in a Larger Context
Conclusion
Glossary
Appendix: Quotations from the Scientific Literature
Notes
Bibliography
Index
ILLUSTRATIONS
Figure 2.1. Preimplantation human development
Figure 3.1. Differences that can only be detected over time
Figure 4.1. Organization of the amniotic and chorionic membranes determines the timing of twinning
Figure 4.2. A blastocyst hatching from the zona pellucida
Figure 5.1. Twinning by embryo splitting at the blastocyst stage
Figure 6.1. Formation of chimeras and mosaics
Figure 7.1. Embryology as art
TABLES
Table 2.1. Tests for progressive developmental restriction
Table 3.1. Status of entities sharing some features in common with embryos
Table 4.1. Timing of twinning
ACKNOWLEDGMENTS
Portions of this work or the ideas expressed herein have been adapted from previously published texts, including but not limited to the following:
Condic, M. L. “Life: Defining the Beginning by the End.” First Things 133 (May 2003): 50–54.
Condic, M. L., and S. B. Condic. “The Appropriate Limits of Science in the Formation of Public Policy.” Notre Dame Journal of Law, Ethics and Public Policy 17, no. 1 (2003): 157–79.
Condic, M. L., and S. B. Condic. “Defining Organisms by Organization.” National Catholic Bioethics Quarterly 5, no. 2 (2005): 331–53.
Condic, M. L. “When Does Human Life Begin? A Scientific Perspective.” Westchester Institute White Paper (Westchester Institute for Ethics and the Human Person) 1, no. 1 (October 2008): 1–18.
Condic, M. L. “Alternative Sources of Pluripotent Stem Cells: Altered Nuclear Transfer.” Cell Proliferation 41, suppl. 1 (December 2008): 7–19.
Condic, M. L. “Preimplantation Stages of Human Development: The Biological and Moral Status of Early Embryos.” In Is This Cell a Human Being? Exploring the Status of Embryos, Stem Cells and Human-Animal Hybrids , edited by Antoine Suarez and Joachim Huarte, 25–43. New York: Springer, 2011.
Condic, M. L. “A Biological Definition of the Human Embryo.” In Persons, Moral Worth, and Embryos: A Critical Analysis of Pro-Choice Arguments , edited by Stephen Napier, 211–35. New York: Springer, 2011.
Condic, M. L. “The Science and Politics of Cloning: What the News Was All About.” On Point , Charlotte Lozier Institute, May 1, 2013. https://s27589.pcdn.co/wp-content/uploads/2013/05/On-Point-Science-and-Politics-of-Cloning-Condic-May-2013.pdf.
Condic, M. L. “Human Embryology: Science Politics versus Science Facts.” Quaestiones Disputatae 5, no. 2 (2014): 47–60.
Condic, M. L., and K. Flannery, “A Contemporary Aristotelian Embryology.” Nova and Vetera (English Edition) 12, no. 2 (2014): 495–508.
Condic, M. L. “When Does Human Life Begin? The Scientific Evidence and Terminology Revisited.” University of St. Thomas Journal of Law and Public Policy 8, no. 1 (2014): 44–81.
Condic, M. L. “Totipotency: What It Is and What It Is Not.” Stem Cells and Development 23, no. 8 (April 2014): 796–812.
Condic, M. L. “Determination of Death: A Scientific Perspective on Biological Integration.” Journal of Medicine and Philosophy 41, no. 3 (June 2016): 257–78.
Condic, M. L. “The Role of Maternal-Effect Genes in Mammalian Development.” Stem Cell Reviews and Reports 12, no. 3 (June 2016): 276–84.
Condic, M. L. “Embryos and Integration.” In Life and Learning XXVI: Proceedings of the Twenty-Sixth University Faculty for Life Conference , edited by Joseph W. Koterski, 295–323. Bronx: Fordham University Press, 2017.
Condic, M. L. “Virtues beyond a Utilitarian Approach in Biomedical Research.” In Proceedings of the XXII PAV General Assembly , 99–113. Rome: Libreria Editrice Vaticana, 2017.
Condic, M. L., and S. B. Condic. Human Embryos, Human Beings: A Scientific and Philosophical Approach . Washington, DC: Catholic University of America Press, 2018.
NOTE ON THE PRESENTATION OF CITATIONS IN THE TEXT

Some endnotes expand upon points that are stated briefly in the main text or refer readers to other sections of the book that discuss the topic in greater detail. You can identify endnotes that contain additional discussion, clarification, or an internal reference by the brackets flanking their superscripted callout numbers in the text.
INTRODUCTION
Human Embryos and Human Individuals
For the vast majority of human history, prenatal development was a deep mystery that could not be penetrated by direct observation. While scientists, philosophers, and bioeth I cists have considered the origins of human life for a long time (for example, Aristotle discusses embryonic development extensively in De generatione animalium ), the conclusions they have drawn were often based on very little evidence. Consequently, appealing to historical “experts” yields a plethora of opinions, many of which have very little to do with the scientific facts. 1
In modern times, with the advent of chemical contraception, in vitro fertilization (IVF), and human embryo research, determining precisely when human life begins has become a matter of considerable importance. Each of these practices raises significant questions regarding the nature of the entity produced by sperm-egg fusion and society’s obligation to that entity. The ability to manipulate the earliest stages of human life in the laboratory has brought into sharp focus a number of questions that are vital to our understanding of human beings and human rights, including the following: When does human life begin? Is a human embryo a human individual? What is the basis of human value?
There are no universally agreed-upon answers to these questions. Life is clearly a continuum, with living cells giving rise to new types of cells and, ultimately, to mature individuals. This fact has led many to conclude that it is impossible to determine when human life begins and to question whether human embryos have greater value than human cells. Yet this view raises a serious ethical dilemma: while no one objects to the destruction of ordinary human cells for biomedical research, the use of human beings for such purposes is universally condemned. To resolve this dilemma, clear criteria must be established to determine when living human cells give rise to a new individual human being.
The phenomenon of identical (monozygotic) twinning presents a significant challenge to the view that human life and human personhood begin at conception. 2 The fact that a single embryo can split to generate two (or more) genetically identical embryos seems to defy the notion that prior to splitting, the embryo can itself be an individual human being. The fundamental philosophical challenge of twinning is an ontological one; if a one-cell embryo (or zygote) that would normally mature into a single individual can split early in development to give rise to two embryos, this calls the ontological status of the original zygote into question. A single cell cannot be simultaneously one individual and two individuals. Consequently, many have concluded that so long as the potential for identical twinning exists, no single human individual can exist.
The view that no human individual can exist so long as twinning is possible has led to a widespread denial of the individual humanity of early human embryos, particularly within the scientific community. This view of the embryo was initially promulgated in 1979 by biologist Clifford Grobstein, chairman of the biology department and the dean of the School of Medicine at the University of California, San Diego. In defense of the newly pionee